1. Field of the Invention
The present invention relates generally to test equipment, and more particularly to a system and method for verifying central office (CO) wiring associated with line sharing.
2. Background of the Invention
The Federal Communications Commission (FCC) has promulgated rules that require Incumbent Local Exchange Carriers (ILEC) to share certain telecommunications resources with Competitive Local Exchange Carriers (CLEC). One of these rules enables a CLEC to use telephone lines of an ILEC, in competition with the ILEC, to offer telecommunications services to customers of the ILEC. Such line sharing arrangement allows the CLEC to provide, for example, digital subscriber line (DSL) services over the same loop that is used by the ILEC for voice communications.
Without the line sharing arrangement, DSL services can be provided by the ILEC using a combined splitter and DSL modem (also known as a digital subscriber line access multiplexer or DSLAM) that are placed at a common location within a CO. Testing or verification of the wiring would not be a difficult task because both voice and data are provided by the ILEC. Under a line sharing arrangement, however, the CLEC""s DSLAM is a different unit that is physically separated from the ILEC""s splitter. Due to the competitive nature between the ILEC and the CLEC, the ILEC""s splitter and the CLEC""s DSLAM are physically located in different parts of a CO, even though each of the splitter and the DSLAM is ultimately connected to a common telephone line that serves the same customer. More often than not, the ILEC""s splitter and the CLEC""s DSLAM are located on separate floors in a building that houses the CO. Such physical separation of the splitter and the DSLAM creates unprecedented complexity associated with testing CO wiring. In some cases, for instance, five two-wire connections between wire terminals are required. This complexity, of course, increases the potential for wiring errors.
Portable telephone test sets are used extensively in the telecommunications industry to establish temporary communications or test lines for proper operation. These test sets are widely referred to as xe2x80x9cbutt sets.xe2x80x9d The term butt sets is used herein to refer to the portable telephone test sets. As known in the art, voice circuits can be xe2x80x9cverifiedxe2x80x9d or tested using the CO""s embedded Automatic Number Announcement Circuit (ANAC). The verification process typically involves the following steps. First, a technician bridges across the circuit with a conventional butt set. Second, the technician causes the butt set to go off-hook to draw a dial tone. Third, the technician dials the ANAC number. Fourth, the ANAC responds with the telephone number of the telephone line being tested. Fifth, the telephone number provided by the ANAC is used by the technician to verify the line. Unfortunately, this method of verification cannot be used to verify the DSL circuit in a line sharing arrangement in which the splitter and the DSLAM are physically separated.
To minimize the possibility of faults on the DSL circuit affecting the voice circuit, the industry standard for line sharing requires a blocking capacitor in the splitter. As known in the art, the blocking capacitor prevents the flow of direct current, which signals the switch to provide dial tone. For this reason, the ANAC process described above cannot be used.
There are currently no known products on the market that are specifically designed to verify the DSL circuit under the line sharing arrangement. As a result, technicians of local exchange carriers must improvise a method to verify CO wiring associated with line sharing.
Technicians have attempted to verify the DSL circuit using frequencies higher than those in the voice band to overcome the blocking capacitor. This method is undesirable because it requires a transmitter and a specially made receiver that is adapted to detect high frequencies. A conventional butt set cannot be used as the receiver in this method. The use of the transmitter and the special receiver to verify CO wiring is not considered to be cost-effective. In some situations, several special receivers per CO may be necessary, making this solution more expensive and even less desirable.
Accordingly, there is a need for a system and method that can verify the DSL circuit in a line sharing arrangement in a cost effective manner. Specifically, there is a need for a system and method that can utilize existing conventional butt sets to verify the DSL circuit in a line sharing arrangement.
The present invention is a system and method that uses amplitude modulation to verify the DSL circuit in a shared telephone line. The system of the invention includes a transmitter and a receiver. The transmitter sends an amplitude-modulated test signal to the shared telephone line. The test signal is introduced to the telephone line on one side of a blocking capacitor that isolates a DSLAM on the DSL circuit. The test signal is a product of a high frequency carrier signal and a low frequency audible signal. The receiver is connected to the DSL circuit of the shared telephone line on the other side of the blocking capacitor to detect the test signal. The DSL circuit is verified if the test signal is detected by the receiver through the blocking capacitor.
In a preferred embodiment of the invention, a conventional butt set is used as the receiver. It is noted that most conventional butt sets have enough non-linearity to detect the signal. In instances in which newer butt sets (designed with better linearity) are employed, an external detector may be employed. The transmitter has an amplitude modulator that mixes the audible signal received from a low frequency oscillator and a carrier signal received from a high frequency oscillator. The product of the amplitude modulator is the amplitude-modulated test signal.
The carrier signal generated by the high frequency oscillator could have a frequency as low as a few tens of kHz, below which the filtering described below would be difficult. It could have a frequency as high as a few tens of MHz, above which the attenuation of the wiring and cabling through which the signal must pass might be excessive. Preferably, the carrier signal is in the range of about 25 kHz to about 1 MHz, the same range of frequencies used by ADSL. At about 100 kHz, the frequency of the carrier signal is high enough that the signal is transmitted through the blocking capacitor and low enough that the signal is not unduly attenuated by the wiring and cabling through which it must pass.
In the preferred embodiment, the audible signal generated by the low frequency oscillator preferably has a frequency at between about 300 Hz and about 3 kHz. Preferably, the audible signal is at about 1 kHz. Preferably, a gate or a switch is provisioned between the low frequency oscillator and the amplitude modulator to regulate the input of the low frequency signal to the amplitude modulator at a low rate, for example, less than about 5 Hz. For example, the gate opens at a rate of about two times per second. This rate is desirable because it produces a tone that is distinguishable by a technician during verification.
The test signal generated by the amplitude modulator is then processed by a high-pass filter to remove any residual low frequency signals before it is used to verify the DSL circuit. Preferably, the transmitter is also equipped with a low-pass filter to prevent the remaining high frequency components of the test signal from going to the outside plant or the customer side of the telephone line. This allows the customer to use the telephone line for voice communications without interruption. The test signal that goes to the CO side of the telephone line does not go through the low-pass filter in the transmitter. As a result, the high frequency test signal goes from the transmitter to the CO side of the telephone line.
Preferably, the test signal is output from the transmitter to a main distributing frame of the CO to test the telephone line. The test signal goes through the main distributing frame to a splitter having a blocking capacitor and a low-pass filter, which are connected to the DSLAM and a voice switch, respectively. Since the test signal has a high frequency carrier component, the test signal can go through the blocking capacitor to verify the DSL circuit. A receiver placed on the other side of the blocking capacitor then verifies the presence of the test signal on the DSL circuit. The low-pass filter in the splitter ensures that the test signal does not interrupt the voice circuit part of the telephone line.